RFR: 8315801: [PPC64] JNI code should be more similar to the Panama implementation

[Martin Doerr]

This PR removes writing parameters to stack which are passed in registers. (See JBS issue for more details.) This allows us to get rid of the extra parameter for `c_calling_convention`.

We can also remove the extra space for the Parameter Save Area in some ABIv2 cases (ppc64le).

Tests have passed on linux ppc64 and ppc64le and AIX.

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Commit messages:
 - Remove regs2 from c_calling_convention.
 - 8315801: [PPC64] JNI code should be more similar to the Panama implementation

Changes: https://git.openjdk.org/jdk/pull/15599/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15599&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315801
  Stats: 105 lines in 12 files changed: 16 ins; 69 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/15599.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15599/head:pull/15599

PR: https://git.openjdk.org/jdk/pull/15599